The invention relates to an improvement in the electrosurgical electrode which is known as the dispersive, neutral or feedback electrode.
Referring to FIGS. 1 and 4, a known electrosurgical unit (ESU) comprises three main parts: a radiofrequency current (RFC) generator 1, an active electrode 2 and a neutral or dispersive electrode 3.
It is vital in electrosurgery to control the path followed by the current from the active electrode to the dispersive electrode to avoid hot spots that can injure the patient. Immediately at the active electrode there is a first "hot spot", which vaporizes the liquids in the tissue and breaks the tissue down. This, of course, is desirable, as it is the means for performing the electrosurgery. Other hot spots must be avoided. If the current is permitted to pass through the patient's body and concentrate at a localized point, for example where the patient's body contacts a grounded portion of the operating table, the patient can easily be seriously burned. The burn may go undetected because the patient is under anesthesia.
Such burns usually occur at a point where there is high pressure due to the patient's weight. This often occurs at a point where the patient contacts the operating table, or contacts a large plate-like dispersive electrode of the type which is placed under the patient's body. The pressure causes both reduced impedance and reduced cooling due to blood perfusion, whereby the local temperature can go high enough to destroy tissue. Necrosis caused by current leakage is often discovered long after the surgery.
RFC generators commonly employ a non-adjustable frequency of 2.2MHz, which is advantageous for cutting but also entails a great risk of burns at the dispersive electrode.
Various attempts have been made to control the current path by creating a minimum impedance path between the active electrode and the dispersive electrode. Previous solutions have attempted to use a large-area dispersive electrode beneath the patient, or to adhere the dispersive electrode to the patient's skin closer to the active electrode. None of these solutions has been fully successful.
On the one hand, wide-scale electrodes have proved unsatisfactory, because of the fundamental behavior of high-frequency currents. Until now the dispersive electrodes have been designed according to criteria selected to satisfy safety standards, for example, 1 square centimeter per 1.5 watts of radiofrequency power. However, such criteria are inadequate, because until now manufacturers have not taken into account that the RFC is not evenly distributed over the surface of the dispersive plate 5. On the contrary, as indicated by the cross-hatching in FIG. 4, almost all of the current is collected in about 20% of the electrode area, which is found in the peripheral zone 12 of the electrode in contact with the patient's skin. This edge effect can cause locally high current densities and hot spots.
On the other hand, a dispersive electrode on the patient's body close to the active electrode can physically interfere with the surgery, particularly since prior electrodes have been relatively thick, for example 1-2 mm. Surgeons prefer a large, clear surgical field because the surgery often expands unpredictably. For this reason, the known prior art has never succeeded in locating a dispersive electrode close to the surgical site.
One prior arrangement, disclosed in U.S. Pat. No. 4,269,189 to Abraham, includes a circular or oval electrode with a substantial central aperture. An adhesive layer contacts the patient's body both outside the periphery of the electrode and within the central aperture, for obtaining better adhesion. It is intended to be applied to any selected part of the patient's skin. However, this arrangement would interfere with the surgery if it were employed near the surgical field.
Theoretical and experimental studies carried out many years ago by Maxwell, Poisson, Laplace, and others, have shown that the most efficient electrode area for the collection of current from the active electrode is the peripheral portion of the dispersive electrode which is in contact with the patient's skin. See generally M. Aubry-Frize et al., "Assessment of Skin Temperature Elevation and Heat Diffusion with Electrosurgical Currents," Medical Instrumentation, Vol. 14, No. 5, at 272-275 (Sept.-Oct. 1980); J. D. Wiley et al., "Analysis and Control of the Current Distribution Under Circular Dispersive Electrodes," Transac. on Biomedical Engineering, Vol. BME-29, No. 5, at 381-85 (May 1982); and G. R. Newfeld et al., "Electrical Impedance Properties of the Body and the Problem of Alternate-Site Burns During Electrosurgery," Medical Instrumentation, Vol. 19, No. 2, at 83-87 (Mar.-Apr. 1985). The latter reference points out that burns can develop from excessive current density through the dispersive electrode as well as at alternate sites remote from either the active or the dispersive electrode.
These studies of the edge effect enabled Abraham to place a large aperture in the center of the electrode with an exposed adhesive pad therein. However, adhesive electrodes such as Abraham's are commonly used even farther from the surgical site than a plate electrode (under the patient) would be. Also, Abraham's electrode has the usual overall dimensions and shape of a standard electrode. Thus, Abraham's device is still subject to potential hot spots at its edges.
Other disadvantages of Abraham's device are that it covers the entire area with an adhesive pad and a gel pad and cannot breathe. It would physically interfere with the surgery if it were employed near the surgical field.